This invention relates generally to fluid chambers for enclosing electrical devices, and more particularly to safety devices for relieving fluid pressure in a chamber, if the pressure suddenly increases.
In electric devices used to cut off electric current, such as circuit breakers, commutators and disconnecting switches, the electric current path is broken within an air-tight enclosure containing an insulating fluid under pressure such as sulfur-hexafluoride (SF.sub.6). As the current path is broken, an arc is drawn. The arc can produce a sizable over-pressure inside the air-tight enclosure. The over-pressure may result for several reasons, such as failure of the electric arc to extinguish at the time of opening of the contacts; failure of the controlling device resulting in insufficient speed of the movable contacts at the opening or at the closing, or insufficient movement of the movable contacts; and untimely lowering of the pressure of the insulating fluid inside the enclosure due to leaks for example, which result in a diminuation of the insulation between the parts under electric tension.
The over-pressures generated by the aforementioned failures can be significant, particularly if the failures lead to a non-breaking or to short-circuiting between phases. When over-pressures occur, the enclosure can explode, damaging not only the electric device contained in the enclosure and the neighboring equipment, but also injuring personnel in the area. Therefore explosion of the enclosure must be avoided.
In order to reduce the damage resulting from over-pressure, it is known in the art to include one or more safety membranes on the air-tight enclosure of current-breaking devices. The safety membranes are generally made of metallic or carbon disks which are generally connected with the interposition of sealing washers to an opening provided in the wall of the air-tight enclosure. The safety membrane ruptures at about an acceptable maximum fluid pressure at which no danger is presented to the enclosure. However, such safety membranes are not always effective because the pressure which causes their rupture depends to a great extent on the rate of the increase of the pressure and on the elasticity of the membrane. Further, in spite of the presence of sealing washers on both sides of the safety membrane, the seals constitute additional sources of fluid leakage which can result in insulation failure during operation of the electric device.
Air-tight molded resin enclosures are known in which the safety membranes consisting of a flat part are set up in a convex cover and therefore are less resistant to the fluid pressure than the convex parts of the cover. However, this arrangement does not prevent damage to the neighboring equipment in the event that over-pressure causes the membrane to rupture. If the current-breaking device which causes the rupture to occur is located in a cell, the neighboring cells are also damaged.
It is therefore an object of this invention to provide an air-tight molded resin enclosure having a safety membrane which can rupture at a specific pressure, and which ruptures in such a manner as to avoid damage to the neighboring equipment.
According to a preferred embodiment of the present invention, the air-tight enclosure includes a tubular flange molded in one piece with the enclosure. The area of the enclosure at which the flange is joined includes a flat part which forms the safety membrane. The flat part forming the safety membrane has a thickness less than the thickness of the rest of the enclosure. Further, a groove in the shape of a closed loop may be formed by molding all around the safety membrane between the membrane and the base of the external flange. A plurality of grooves could extend from the center of the safety membrane all the way to the closed loop shaped groove. In this way, the rupturing pressure of the safety membrane could be much lower than the rupturing pressure of the enclosure. Further, this arrangement is especially attractive when using fragile material having a low elasticity limit because the speed of propagation of the rupture is high. Additionally, if the safety membrane ruptures under internal over-pressure, the external tubular flange permits the fragments of the safety membrane as well as the hot fluid under pressure which has caused the rupture to be directed in a pre-determined direction thereby preventing danger to personnel and neighboring equipment.
According to another embodiment of the present invention, the external tubular flange may be provided on its external end with a connecting clamp molded in one piece with the enclosure and the flange. In this embodiment the flange may be attached through its connecting clamp to a tubular leg which may be used both as a support for the air-tight enclosure and as a conduit for the evacuation of the hot-fluid under pressure in case of rupturing of the safety membrane.